In optical phase modulation, the efficiency of frequency utilization per bit rate is high. PSK (Phase Shift Keying) and DPSK (Differential Phase Shift Keying) are well known as such optical phase modulation in the art.
In complete modulation, it is necessary to dispose an optical phase locked loop (PLL) circuit for regenerating an optical carrier on a receiving side and moreover an optical Costas loop is required for the optical PLL. An optical PLL in an optical receiver in homodyne detection is described in “Balanced phase-locked loops for optical Homodyne receivers: Performance Analysis, Design Considerations, and laser Linewidth Requirements” by Leonid G. Kazovsky, Journal of lightwave technology, Vol. LT-4, No. 2, February 1986 and “10 GBIT/S 2-PSK TRANSMISSION AND HOMODYNE COHERENT DETECTION USING COMMERCIAL OPTICAL COMPONENTS” by Stefano Camatel et al., ECOC 2003, Vol. 3, We. P. 122, pp. 800-801. Because of the above-mentioned reasons, incomplete modulation is generally used in which optical carriers resides. The contents of the above papers are incorporated in this specification for reference.
In the incomplete modulation, as an amount of residual carriers increases , a level of PSK signals becomes lower, and accordingly, a signal to noise ratio (SNR) also deteriorates. Under such circumstances, an optical receiver is forced to operate an optical PLL depending on a small amount of optical carriers and therefore demodulating operation becomes unstable.